The measurement of light absorption and/or scattering when propagating through or reflecting from a certain medium forms the basis of a number of optical spectroscopic methods widely applied in various medical domains, such as patient monitoring. An illustrative example is transmissive pulse oximetry.
Pulse oximetry is an optical method for non-invasive monitoring of arterial oxygen saturation of a patient and has become one of the most commonly used technologies in clinical practice. The protein haemoglobin (Hb) binds oxygen in the red blood cells for transport through the body, and has the property of changing from dark red to bright red in colour when oxygenated. By emitting and detecting light at two or more wavelengths, pulse oximeters determine the light absorbance in a peripheral vascular bed to arrive at an indirect estimate of oxygen saturation, i.e. the concentration fraction of oxyhaemoglobin (HbO2). Pulse oximeters rely on the changes in arterial blood volume caused by cardiac contraction and relaxation to determine the amount of light absorbed by pulsating arterial blood alone, thereby largely factoring out the contributions of tissue and venous blood.
In many applications, including oximetry, simultaneous or quasi-simultaneous attenuation measurements of an optical path at different wavelengths, i.e. of different colours, are required. To that end, typically multiple light sources are utilized which are generally combined with a single photo-detector. In order to be able to distinguish between the signals from each of the emitters at the photo-detector, in general, electrical multiplexing methods are employed, such as time division multiplexing (TDM), frequency division multiplexing (FDM), or code division multiplexing (CDM).
In the medical practice, light attenuation measurements applied in e.g. patient monitoring suffer from electromagnetic interference. Typically such interference consists of ambient light at various optical wavelengths and with different modulation frequencies. Common examples include natural daylight, which is typically not modulated, as well as artificial light from incandescent lamps, which is modulated at the double mains frequency (100 Hz or 120 Hz) and 50 Hz or 60 Hz harmonics, and from fluorescent lamps with flicker rates ranging from tens to hundreds of kilohertz depending on the specific electronic ballast.
Generally, in spectrometric devices measures are taken to mitigate the effect of external interference on the measurements. For example in pulse oximeters, the light sources are modulated such that at the photo-detector the emitted light can be distinguished from ambient light by filtering or demodulation. Regardless of the modulation techniques applied, conventional methods rely on knowledge of the spectral modulation of the environmental light and assume that the light source modulation frequency or band that is used can remain fixed for the lifetime of the device. However, if the ambient light modulation spectrum is only partly or not known a priori, such as is the case when the spectrometric device operates in the vicinity of light communication systems, then interference may be present in the modulation spectrum of the detected light at the device operation frequency. Similarly, new operation schemes of high-intensity discharge (HID) lamps might result in interference signal with a wide frequency range. Moreover, emerging light emitting diodes (LEDs) light sources are foreseen to use a wide range of modulation frequencies, creating new sources of interferences. If an interferer contaminates the operation frequency band, the signal-to-interference ratio (SIR) may decrease to a large extent, thereby degrading the measurement quality.